Define an instance for BluetoothSerial as a global variable, serialBT.
To set, we use begin() function having a parameter indicates the name of the Bluetooth device for the Bluetooth scanners. We named that ODROID-GO.

And in loop() function, if a message from the Serial monitor available, send that message to the connected device via Bluetooth serial.
On the other hand, if a message from the Bluetooth serial available, send that message to the host.

Serial Monitor

You can use a Serial monitor to watch the debug messages from the Serial port.
It is placed at the Tools → Serial Monitor menu to open. Or, you can press CTRL-SHIFT-M to open easily.

To show the message properly, you should select bandwidth to 115200 baud.

This would be very helpful debugging tool for you.
In this guide, we're using this tool to communicate with the connected device.